Capacitor-discharge device for blasting



NOV-3,1970 F. B. JANOSKI I ,538,4

CAPACITOR-DISCHARGE DEVICE FOR BLASI'I'ING I Filgd July 1. 1968 ATTORNEY United States Patent 3,538,414 CAPACITOR-DISCHARGE DEVICE FOR BLASTING Florian B. Janoski, Havertown, Pa., assiguor to Atlas Chemical Industries, Inc., Wilmington, Del., a corporation of Delaware Filed July 1, 1968, Ser. No. 741,402 Int. Cl. H02m 3/00 US. Cl. 320-1 11 Claims ABSTRACT OF THE DISCLOSURE A capacitor-discharge blasting device is disclosed having a capacitor drain means connected for limiting the voltage across the capacitance to a negligible level during periods of non-use. The present capacitor-discharge blasting device preferably includes the capacitor drain means in combination with a capacitor capable of delivering an energy output of between about 210 to about 400 wattsecouds of energy over a relatively short period of time, and a voltage sensing switch for releasing energy from the capacitor when the required level of energy output is available. The output energy although sufficient to initiate up to about 150 blasting caps disposed in a parallel circuit is insufiicient to cause malfunction by internal arcing of as few as one electric blasting cap disposed in the parallel circuit.

This invention relates to a capacitor-discharge blasting device having a capacitor drain means connected for limiting the voltage across the capacitance to a negligible level during periods of non-use, for safely and conveniently aborting a blast at any time prior to energizing of the external blasting circuit and, more particularly, to such a device for initiating as many as about 150 electric blasting caps disposed in a parallel circuit arrangement without malfunction by internal arcing of as few as one electric blasting cap.

In certain blasting operations such as those performed in tunnels and shaft mining, it is generally desirable to connect electric blasting caps in a parallel circuit, principally because such a circuit permits rapid connection of the blasting caps with minimal possibility of error. Typically, the electric blasting caps are initiated with electric energy received from a readily available AC power line.

Malfunctioning of electric blasting caps and in particular those of the delay type may be attributed to the large amount of heat generated by an electrical arc which occurs within an electric blasting cap when an excessive elec trical current is supplied. Although it is recognized that excess current may be usefully channeled in the parallel circuit by inclusion of additional blasting caps, increased cost, delay, and inconvenience result unless a simple device is available which is operable for initiation of either as few as one or as many as about 150 blasting caps disposed in a parallel circuit.

The various malfunctions caused by arcing occur more frequently when any of the various parallel circuit arrangements such as normal parallel, reverse parallel, and closed loop parallel are used with a continuous source of electrical energy of high potential such as a power line. When electric blasting caps are connected in parallel, the votage across the bridgewire terminals frequently is high enough to sustain an electrical are after the bridgevvire melts; and, when a power line of high current capacity Patented Nov. 3, 1970 is used, the arc may generate enough heat within the blasting cap prior to initiation to cause malfunction. Parallel circuit arrangements of delay detonators are often used in the blasting art, and the convenience and advantages of a power line as a source of electrical energy are well known. Thus, the majority of arcing difiiculties encountered in blasting operations occur when a commonly used circuit arrangement is combined with a commonly used source of electrical energy.

One approach to solving difficulties encountered in initiating electric blasting caps disposed in a parallel circuit is disclosed by McFarland in US. Pat. No. 3,056,349. McFarland teaches a blasting circuit for initiating, in parallel arrangement, delay electric detonators by generally applying firing energy insuflicient to cause malfunction of detonators by internal arcing. Firing current is received from a continuous source of electric enegy, and a quick current-interrupting switch is employed to restrict the time interval over which the current is applied.

Another approach that has been tried in an attempt to eliminate the arcing problem when firing delay electric caps in parallel is to use a commercially available condenser-discharge blasting machine as the power source. Such blasting machines have been disclosed by Wolf in US. Pat. No. 2,892,128 and Bickel et al, in U.S. Pat. No. 2,908,847. This solution suffers from the disadvantage that these blasting machines can fire only a limited number of caps connected in parallel 'while avoiding malfunctioning when as few as only one blasting cap is initiated.

Various other approaches for limiting malfunctioning by arcing of electric blasting caps disposed in a parallel circuit such as limiting the magnitude of the firing current and reducing the resistance inside the electric blasting caps have realized limited success. These approaches have attempted to limit the energy available for initiating electric blasting caps disposed in a parallel circuit based on the fact that energy is a direct function of the square of the firing current, times the internal resistance of the cap, times the duration of time the current is applied.

There are several reasons why various prior art methods of initiating from one up to about blasting caps disposed in a parallel circuit by a single device have not been completely successful in eliminating arcing malfunctions. One reason is that some of these methods attempt to control only one of the three contributing factors that determine the amount of heat generated inside the cap. If one, or both, of the other contributing factors become excessive, the limiting effect of the one being controlled may be overridden. Another reason is that other solutions depend on a human decision or action. Hence, human error easily negates their effectiveness.

One of the other disadvantages typically present in commercially available condenser-discharge blasting machines is that the condenser may retain an electric charge during periods of non-use. A dangerous situation is thus presented should electric blasting caps be connected such that the capacitor is prematurely discharged into the blasting circuit to accidentally fire the caps. One attempt to provide a capacitor drain in an effort to avoid premature discharge of a capacitor in a blasting device is disclosed by Bickel et al., in U.S-. Pat. No. 2,908,847. Bickel et al., teach the inclusion of a resistor between the output wires for draining the circuit when the firing switch and ready switch are pressed and held in the fire and ready positions, respectively. Upon release of either of the switches, the resistor is disengaged from performing its task as a bleed-off path and a measurable voltage will remain on the capacitor for a considerable time. Another ditficulty in such an approach is that, since the capacitor bleeding resistor remains permanently connected in the output circuit, it affords opportunity for draining energy from the circuit which would otherwise be effectively applied to initiating electric blasting caps.

Heretofore, no commercially available blasting machine has been available which is rated to fire more than about 50 blasting caps connected in a parallel circuit and which has an efiicient capacitor drain means connected for limiting the voltage across the capacitance to a negligible level during periods of non-use. This situation is unsuitable because operators employing parallel circuits of blasting caps desire to safely employ more than 60 caps and many would desire to employ more than 100 caps in a single shot by a device which is also capable of initiating as few as one blasting cap without circuit modification and malfunctioning problems.

It has now been found that by the practice of the present invention from one to about 150 electric blasting caps may be initiated in a parallel circuit arrangement without encountering arcing malfunctions when a firing switch is actuated, and the capacitor may be drained for limiting the voltage across the capacitance to a negligible level during periods of non-use. The device presented requires only the decision to fire the blast; and, upon activation, the necessity for further decision or action with regard to the device is eliminated. Misfires due to human error in premature functioning of the device are eliminated. The present invention uses output energy as a function of the capacitance and voltage of a condenser-discharge blasting device.

Generally stated, the present invention provides a blasting device for initiating up to about 150 electric blasting caps disposed in a parallel relation by combining an energy output means capable of delivering between about 210 to about 400 watt-seconds of energy over a relatively short period of time from a capacitor having a preferred capacitance of about 28x10 farad to about 5.0x 10* farad. A voltage sensing switch is also desirably included in the blasting device for sensing the limit of the voltage required for charging the capacitor and, after the voltage level for charging the capacitor is obtained, for discharging the capacitor and providing the discharged energy to initiate the electric blasting caps of the parallel circuit. Also included in the combination of elements in the present device is a capacitor drain means connected for promptly limiting the charge on the capacitor to a negligible level during periods of non-use. Thus, by using a voltage sensing switch and incorporation of the prompt capacitor drain means, the present blasting device may be safely operated with a greatly reduced opportunity for human error. In an embodiment of the present invention, other voltage sensing means may be employed in the circuit to accomplish equivalent performance.

The invention will be further described in connection with the accompanying drawing which illustrates schematically a circuit which may be used in the practice of the present invention.

The figure illustrates a blasting device prepared by the practice of the present invention. Terminals 4 and 6 are illustrated for receiving an alternating current from, for example, a power line, an alternating current generator, or other source capable of supplying an AC voltage sufficient to charge the capacitor system described hereinafter. A standard 115 volt AC outlet is generally sufiicient. Connected between lines and 7 is a power-on indicator light 8, exemplified by a NE-S 1H neon indicator lamp, in series with resistor 10, exemplified by a 47,000 ohm, 0.5 Watt, wire wound resistor. -Line 5 joins one terminal post 12 of switch 14, and line 13 joins a second terminal post 16 of switch 14. Switch 14, exemp fied by a sp ing return,

double-pole, double-throw toggle switch having a rating of 6 amperes and 125 volts, is illustrated in normal circuit position when connected to terminal posts 18 and 20. Line 21 electrically joins terminal posts 1 8 and 20 of switch 14. Line 19 from terminal post 18 is electrically joined to resistor 22, exemplified by a 500 ohm, 50 watt, wire wound resistor; and line 21 from terminal post 20 is electrically joined to bridge line 23-. Bridge line 23 connects into bridge rectifier 24 formed, for example, of four diodes 26, exemplified by General Electric IN506'2. silicon diodes. Adjacent diode connection lines 25 and 27, to that of bridge line 23, receive an electric current from the high potential side of a step-up transformer 28, exemplified by a Thordarson-Meissner 24R100 transformer. Electric current to the low potential side of transformer 28 is received from lines 7 and 13 when switch 14 is in position providing a closed circuit across terminal posts 12 and 16.

Corresponding to bridge line 23 for receiving rectified current from bridge rectifier 24 is bridge line 29 electrically joining resistor 30, exemplified by a 1,000 ohm, watt, wire wound resistor, which joins line 31 illustrated connected to charging position 32 of power relay switch 34. Charging of energy storage capacitor 38 can only take place when switch 14 is in position providing a closed circuit between terminal posts 12 and 16. Illustrated in phantom is switch 34 when connected to capacitor discharging-to-output position 36. Line 37 receives current from capacitor discharging position 36 when discharging capacitor 3 8 for releasing energy to terminals 40 and 42 by lines 23 and 37. The supplied energy to terminals 40 and 42 may be received by a circuit having from one up to about 150 electric blasting caps disposed in parallel circuit arrangement.

Power relay 34 usefully includes a switching coil 44 electrically positioned between lines 39 and 41. An example of a usefully employed power relay is that commercially available from Rowan Controller Co. under the designation B321S81 having a volt AC coil.

Power relay 34 forms a capacitor discharging element of a voltage sensing circuit, and auxiliary relay switch 46 forms the voltage sensing element thereof. Auxiliary relay 46 is exemplified by that available from Potter and Brumfield, identified generally as a plate relay and illustrated in combination with variable resistor 48 to obtain a current sufiicient to energize, and close contacts 52 and 54 of auxiliary relay 46 when the voltage across capacitor 38 is about 450 volts DC.

Connected to power relay 34 is line 33 joining capacitor 38 and line 43 from resistor 22. Capacitor 38 is further connected to line 23 by line 45 to complete the circuit connection. Capacitor 38, disposed between lines 33 and 45, may be formed of eight 400 microfarads, 500 working volts DC electrolytic capacitors; i.e., a total capacitance of about 3.2 10- farad.

The voltage across terminals 4 and 6 for charging capacitor 38, although illustrated in the drawing to be received from an AC source, may be received from a DC power line, battery, DC generator, or the like provided that suificient DC voltage of a magnitude of about 400 to about 500 volts and preferably about 450 volts is available for charging capacitor 38. In this case, the transformer and rectifier circuit would be eliminated and the DC voltage applied to terminals 4 and 6 would be presented from terminal 12 through switch 14 to line 29, and from terminal 6 to line 23. Coil 44 of power relay 34 would be specified for 80 volt DC operation with an appropriate series resistor, and resistor 10 would have an increased value. The charged capacitor upon discharging provides about 210 to about 400 watt-seconds of energy over a period of time within about 0.025 second to initiate from one up to about electric blasting caps disposed in a parallel circuit arrangement.

Usefully disposed in parallel to capacitor 38 is voltmeter 50 connected by line 51 to line 43 and by line 53 to line 23. Voltmeter 50, exemplified by a 0 to 500 DC voltmeter, is usefully included for visually reading the potential applied to capacitor 38. A simple indicator lamp charge of a magnitude in the amount indicated from a capacitor having a rating also indicated.

which may light when the required potential is applied to the capacitor may also be used if desired.

In operation, the blasting device receives current from a conventional outlet by terminals 4 and 6. When switch 14 is in the illustrated normal position, the transformerrectifier charging circuit is not energized and a drain of energy stored on capacitor 38, and reduction of voltage to the zero level, is effected through resistor 22 and connecting lines 19 and 21 which is a closed circuit with lines 23 and 45 on one side of capacitor 38 and lines 33 and 43 on the opposite side thereof. A safety feature thus results in the capacitor drain circuit in that the normal position of the spring return toggle switch provides that resistor 22 is electrically across capacitor 38 and that the capacitor is not charged during non-use of the blasting device. The capacitor drain feature also provides a convenient means to safely abort initiation of blasting caps at any time during the period of capacitor charge build-up and prior to capacitor discharge to the output terminals. Switch 14 is simply released, and any stored charge is drained through resistor 22.

When switch 14 is advanced to terminals 12 and 16, current is permitted to flow through lines 13 and 7 to the low potential primary winding side of transformer 28. On the high potential secondary winding side of transformer 28, current is received through lines 25 and 27 by bridge rectifier 24 which converts alternating current to direct current for charging capacitor 38 by lines 23 and 31 when power relay 34 is in the capacitor charging position 32. Resistor limits the current surge and forms a :voltage divider with resistor 48. As the charge builds up on capacitor 38, the trickle current through resistor 48 and the coil of auxiliary relay 46 increases. The current required to energize the coil of auxiliary relay 46 is set, by adjustment of variable resistor 48, to coincide with the desired voltage level on capacitor 38. When this current level is reached, terminal 52 is connected with terminal 54 and makes available alternating current to line 55 and line 41 for energizing coil 44 of power relay 34. Power relay 34 is thus caused to rapidly switch to capacitor discharging position 36 for discharging the energy of capacitor 38 through lines 37 and 23 providing energy at terminals 40 and 42 for initiating up to about 150 blasting caps disposed in a conventional parallel circuit, not shown.

Conveniently disposed between lines 37 and 23 is dis charge indicator 56, exemplified by a NES 1H neon lamp in series, with a resistor 58, exemplified by a 47,000 ohm, 2 watt, wire wound resistor, positioned along line 59. Discharge indicator 56 usefully visually indicates when capacitor discharge is effected.

The invention is further illustrated by the following example:

EXAMPLE Delay electric blasting caps in the number indicated in the following table were connected according to standard blasting procedures in a parallel circuit arrangement and initiated by a blasting device having an energy dis- -It is apparent from the foregoing example that the present blasting circuit releases energy from a capacitor sufiicient to initiate up to about electric blasting caps disposed in a parallel circuit arrangement while being an energy level insufiicient to cause malfunction of any one electric blasting cap disposed in a parallel circuit by internal arcing.

It is understood that the foregoing detailed description is given merely by way of illustration and that many variations may be made therein without departing from this invention.

What is claimed is:

1. A blasting device for firing 1 to about 150 bridgewire-type electric blasting caps in parallel comprising (a) capacitor means capable of discharging between about 210 and about 400 watt-seconds of energy,

(b) means for automatically discharging said capacitor means into the firing line of said electric blasting caps when said capacitor means has been charged suificiently to discharge about 210 to about 400 wattseconds of energy,

.(c) capacitor [drain means, and

(d) means for connecting said capacitor drain means to said capacitor means once said capacitor means has discharged and for disconnecting said capacitor drain means from said capacitor means when said capacitor means is being charged.

2. The blasting device of claim 1 wherein a single switch means is provided which connects said capacitor drain means to said capacitor means in its off position and disconnects said capacitor drain means from said capacitor means and charges said capacitor means in its on position.

3. The blasting device of claim 2 wherein said switch means is spring-biased to its 01f position.

4. The blasting device of claim 1 wherein said means of element (d) can additionally be made to connect said capacitor drain means to said capacitor means during the time that said capacitor means is partially charged.

5. The blsting device of claim 1 wherein said means for automatically discharging said capacitor means is an electro-rnagnetically operated unit.

6. A blasting device for firing 1 up to about 150 bridgewire-type electric blasting caps in parallel comprising (a) capacitor means having a capacitance of about 2.8 10" farads to about 5.0 1O- farads,

(b) voltage sensing means for sensing a voltage between about 400 and about 500 volts across said capacitor means,

(c) means for automatically discharging said capacitor means into the firing line of said electric blasting caps when said voltage sensing means senses a voltage of between about 400 and about 500 volts across said capacitor means,

(d) capacitor drain means, and

(e) means for connecting said capacitor drain means to said capacitor means once said capacitor means has discharged and for disconnecting said capacitor means to said capacitor means in its oil position and disconnects said capacitor drain means from said capacitor means and charges said capacitor means in its on position.

8. The blasting device of claim 7 wherein said switch means is spring-biased to its off position.

9. The blasting device of claim 6 wherein said means of element ((1) can additionally be made to connect said capacitor drain means to said capacitor means during the time that said capacitor means is partially charged.

References Cited UNITED STATES PATENTS 2,961,583 11/1960 Sorensen 317-80 3,108,178 10/ 1963 Kelemen et al 3201 X 3,171,011 2/1965 English 219113 -X 3,437,902 4/ 1969 Jones 320 1 TERRELL W. FEARS, Primary Examiner 

